1. Field of the Invention
This invention pertains in general to aircraft engine instruments and more particularly to an apparatus and method for rotating the scale of such instruments to provide alignment of the pointers for a particular thrust setting in order to enable detection of erroneous readings.
1. Description of the Prior Art
Aircraft turbine engine performance parameters such as engine pressure ratio, EPR, exhaust gas temperature, EGT, and revolutions per minute, RPM, are commonly represented on round gauges wherein the reading is obtained by noting the position of a pointer symbol with respect to a scale which is displayed around the circumference of the circular dial. Depending on the particular thrust setting selected, the angular position, or clock position, of the needle or pointer with respect to the circular scale for one engine parameter, such as EPR, may or may not coincide with the angular position of the pointer with respect to the scale for another performance parameter such as EGT.
Various color bands and marker bugs have been used to indicate the desired reading of a particular performance parameter for a given thrust setting, such as a bug for marking the target EPR setting for takeoff. In order to determine engine performance using these circular dial instruments separate readings must be made for each performance parameter with respect to its corresponding scale. Under certain failure modes an erroneous reading from one performance parameter may not be recognizable without specific detailed reference to the remaining readings with respect to their corresponding scales. During critical flight modes such as takeoff, the time required to discern an erroneous engine parameter reading may be sufficient to cause a delayed reaction to the erroneous reading, and in some cases may be a significant cause of an aircraft accident.
It would therefore be desirable to have an aircraft engine instrument system wherein the angular position, or relative clock position, of the needles or pointers for the various aircraft engine performance parameters are aligned to be the same relative clock position for a particular thrust setting, thereby allowing an early detection of a failed engine or erroneous reading by a quick glance at the needles which should be aligned under normal conditions.